It looks like discoloration and the cause could be many reasons. First try power-washer and after wet the concrete with hot water and use a stiff brush and scrub a small area and see if it will lighten up. If it does keep wetting and scrubbing.

If that don't work, I would call a concrete company because a mild acid solution might need to be used, something like 1%-3% of muriatic acid. They will usually start with 1% and see if it works. You can't use to strong of a concentrate because acid will remove concrete if the mixture contains to much acid so a by starting with a weaker solution and that usually enough to remove discoloration.

This acid is very strong and "potent" so you have to wear protective clothing is why a professional company should be called and let them handle this.

Good luck.